1. Field of the Invention
The present invention relates to a permanent-magnet stepping motor having a permanent-magnet rotor mounted in a stator which is fabricated by mounting annular coils in annular portions surrounded by stator yokes.
2. Description of the Prior Art
A typical permanent-magnet stepping motor comprises two stator units, each of which includes two stator yokes and an annular coil disposed between the yokes. Each stator yoke has magnetic-pole teeth which are regularly spaced from each other and protrude along the axis of the rotor within a cylindrical space formed along the axis of the rotor within a cylindrical space formed around the axis of the rotor. The pole teeth of one stator yoke of each stator unit are arranged in a staggered relation to the pole teeth of the other stator yoke. The two stator units are coupled together at a position where their pole teeth are displaced by an electrical angle of 90.degree., in order to form the stator. A rotor of a permanent magnet is mounted inside the stator.
Each stator yoke is made from a ferromagnetic material. Generally, each stator yoke is stamped out of a steel sheet, and a number of magnetic-pole teeth are formed integrally with the inner surface of the yoke. The stamping produces a mechanical distortion, deteriorating the magnetic characteristics. For this reason, the stator yoke may be later heat-treated to compensate for the magnetic characteristics. A coil is mounted between the two stator yokes, forming one stator unit. As an example, one of the stator yokes is shaped like a disk, while the other is shaped like a cup. These two yokes are coupled together about an axis, in order that their magnetic-pole teeth are staggered, and that the outer yokes of the two units substantially form a closed magnetic path. An end plate and a flange plate, each having a bearing, are mounted on opposite sides of the stator. The rotor inside the stator is rotatably held by the bearings.
Heretofore, a stator of this kind has been assembled by successively aligning the stator yokes and then welding, caulking, or otherwise fastening them. More specifically, the joint between the stator yokes is provided with protrusions and holes or notches. A circumferential alignment is effected with these protrusions and holes, and then two stator yokes are stacked on each other and coupled together. Subsequently, other stator yokes are stacked on each other and coupled together. Therefore, when the fastening operation such as a welding or caulking is performed, the inner magnetic-pole teeth may be deformed. In addition, whenever a stator yoke is stacked, error accumulates, deteriorating the dimensional accuracy, especially the radial dimension. As a result, the motor characteristics vary from product to product. Further, a multiplicity of components including the stator yokes are successively placed in position and stacked. After each component is stacked, it is required to be rigidly fixed. As such, there are a number of fixed locations. Thus, the assembly cannot be carried out efficiently. Especially, when welding operations are effected, splashing welding debris may remain inside the assembly. In this case, after the motor is assembled, it may rotate incorrectly because of the debris.